Gyroscopes have been used to measure rotation rates or changes in angular velocity about an axis. A basic conventional fiber optic gyroscope (FOG) includes a light source, a beam generating device (e.g., a beam-splitter), a coil of optical fiber coupled to the beam generating device that encloses an area, and a light detector. The beam generating device transmits light beams originating from the light source into the coil of optical fiber, and these light beams propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction through along the core of the optical fiber. After propagating through the coil, the two counter-propagating (e.g., CW and CCW) beams are combined and directed to the light detector by the beam generating device. When the FOG is rotated about an axis, the CW and CCW beams experience different pathlengths while propagating through the coil, and the difference between the two pathlengths produces a phase difference between the two counter-propagating beams that is proportional to the rotational rate.
Many FOGs utilize a glass-based optical fiber to conduct light along a solid core of the fiber over long distances with low loss and distortion. This optical fiber has a glass/silica core surrounded by a plastic jacket, or buffer, and may be wound into a cylindrical structure, such as a coil, and affixed to a coil-supporting structure, such as a cylindrical hub, to form a sensing coil. The hub and fiber optic coil are both substantially cylindrical structures oriented about a center axis, and the hub has a relatively smaller radius than the radius of the fiber optic coil. An adhesive coating between the outer surface of the hub and inner surface of the fiber optic coil may be used affix the fiber optic coil to the hub.
The glass/silica core and the plastic buffer of the optical fiber may each respond differently to a variety of environmental factors and thereby adversely affect the pathlength difference between the two counter-propagating waves. The sections of fiber in the coil that are closest to the beam generating device are typically the most sensitive to environmental factors. Some of these environmental factors include temperature and mechanical strain. In this event, the output of the sensing coil yields a phase difference between the two counter-propagating waves that is indistinguishable from a rotation-induced phase difference (i.e., a bias error).
One proposed technique for minimizing this bias error is to wind the sensing coil fiber in a pattern symmetric with respect to the mid-point of the optical fiber length. A variety of winding patterns have been developed having symmetry to the mid-point of the optical fiber length. In general, these winding patterns position the mid-point of the optical fiber length at the inner radius of the cylindrical sensing coil and locate the first and second ends of the optical fiber at the outer radius of the cylindrical sensing coil. A bobbin and/or adhesive may affix the fiber wound in this pattern and leave free pigtails (e.g., a relatively short length of the first and second ends of the fiber) for routing to other components in the optical circuit (e.g., beam-splitter). Despite applying these winding patterns to sensing coils, some environments continue to produce thermally induced strains in these sensing coils that cause temperature sensitivity.
During operation, a FOG may be placed in an environment having a fluctuating ambient temperature. Temperature variations affect the sensing coil because the sensing coil undergoes mechanical strain as a result of a differential thermal expansion. A Coefficient of Thermal Expansion (CTE) mismatch between the glass/silica core and the plastic buffer may result in an axial expansion of the fiber optic coil that is significantly larger than the circumferential expansion of the fiber optic coil. Because of the non-isotropic structure of the fiber optic coil, the circumferential expansion of the fiber optic coil, constrained by the glass/silica core of the optical fiber, is significantly smaller than the axial expansion of the fiber optic coil that is dominated by the large CTE of the plastic buffer. Additionally, the glass core generally constrains circumferential expansion of the plastic buffer and forces the plastic buffer to radially expand, and the radial expansion of the plastic buffer may affect the expansion or contraction of adjacent coil layers. Further, the outer diameter of the fiber optic coil generally expands radially away from the center axis of the fiber optic coil while the inner diameter of the fiber optic coil generally expands radially toward the center axis of the fiber optic coil.
Accordingly, it is desirable to provide a winding pattern for a sensing coil in a fiber optic gyroscope that minimizes the temperature sensitivity of the sensing coil from thermally induced strains. Additionally, it is desirable to provide a method for winding a sensing coil for a fiber optic gyroscope that minimizes the temperature sensitivity of the sensing coil from thermally induced strains. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.